Emissions standards throughout the world seek to reduce NOx emissions from automobile emissions, for example, from diesel exhausts. Treatment of NOx can be achieved by the use of a NOx abatement technology such as a lean NOx trap or Selective Catalytic Reduction (SCR) using a reductant. In SCR, a reductant such as hydrocarbon, ammonia or urea-based reagents are used in the presence of a catalyst having a base metal where the reductant reacts to reduce the oxides of nitrogen. The reductant is typically provided by an injector upstream of the SCR catalyst. For systems that use ammonia or ammonia precursors, the use of urea is desirable because it is safer to handle and store compared to the use of ammonia directly. In order to use urea as an ammonia precursor, conditions that promote urea's decomposition into NH3, CO2, and H2O are needed. There are many ways urea can be provided to an SCR of a diesel exhaust system, including those set out in, for example, U.S. Pat. No. 5,601,792 (Hug) and U.S. Pat. No. 5,968,464 (Peter-Hoblyn). In U.S. Pat. No. 6,074,619 (Schoubye), droplets of reductant are provided in an exhaust gas that passes through a series of stacks of parallel plates. The plates of Schoubye can be coated with materials such as oxides of Al, Fe, Ti, Zr, Si, Zn, or Mg to catalyze the decomposition of urea. Further treatment of diesel exhaust emissions include the use of soot filters, a process which requires periodic regeneration of the filter to burn. During regeneration, the steel components of the exhaust system, for example, the exhaust manifold, the exhaust conduit, the ammonia or urea injector, and/or the SCR catalyst, can be subjected to temperatures in excess of 600° C. Other operating conditions, such as heavy loads exerted on the engine at high altitudes, can also result in temperatures in excess of 500° C. in the exhaust system.
Such thermal conditions can activate the metal/steel surfaces of the exhaust system to make them promote oxidation reactions. This causes ammonia intended for use with SCR to degrade. For example, oxides of the steel components, such as chromium (Cr), manganese (Mn), and nickel (Ni) can oxidize ammonia which leads to a change in stoichiometry in the SCR catalyst.
There is a need, therefore, to inhibit reductant decomposition and oxidation in emission systems that utilize NOx abatement catalysts.